This invention relates to a method of manufacturing articles by a permeable mold casting process, wherein computational procedures are utilized to optimize parameters related to the casting process.
A common practice used to manufacture shaped metallic articles is the casting process, wherein a molten metal is poured into a mold cavity of the desired shape, solidified, and then taken out from the mold. In the production of castings having complex geometry, it is further a common practice to form the mold and cores from sand, which is bound together by inorganic or organic binders such as clays and polymeric resins. Since binders are partially decomposed when exposed to a molten metal at an elevated temperature, the shaped casting can easily be removed from the mold.
Optimizing the process parameters of casting brings about the success of manufacturing high-quality articles. For general casting systems, such parameters include mold shape, mold material, runner shape, and other process variables. Some special process variables should also be included in the specific casting method.
In some cases, it is difficult to cause the molten metal to flow through a series of long thin passages without having the liquid freeze prematurely, causing what is referred to as a "cold shut". In other cases, it is desirable to minimize the volume of metal remaining in the runners used to transport the molten metal from a reservoir to the mold cavity. When either of these considerations is important, recourse is often made to vacuum-assisted casting processes. In such processes, vacuum is applied to a portion of the exterior surface of a permeable sand mold, creating a secondary vacuum of reduced magnitude in the mold cavity, which in turn serves to pull the molten metal into the mold cavity through the runners.
The effectiveness of the vacuum casting processes depends on the mold design, the materials used to make the molds and the processing variables, like other casting methods, and specially depends on the vacuum level. A higher vacuum level applied to the exterior surface of the mold increases the vacuum generated in the mold cavity, which in turn increases the velocity with which the molten metal flows into the cavity. This reduces the amount of time required to fill the cavity and hence reduces the probability of forming cold shuts. However, the increased velocities may result in flow patterns which increase the likelihood of gas entrapment.
For similar reasons, the selection of mold materials is also important. In casting processes where vacuum is not used, a coarse sand will increase the permeability of the mold, thus reducing back pressures in the mold cavity and increasing the rate at which the metal fills the cavity. In vacuum casting processes, a coarse sand will help increase the vacuum levels generated in the mold cavity, with a similarly beneficial effect on fill rates. On the other hand, vacuum casting processes might also use a fine sand of low permeability to cover portions of the mold surfaces where gas might leak into the sand mold from the surrounding atmosphere, again effectively increasing the vacuum generated in the mold cavity.
Previously, the method used to select processing parameters requisite to manufacturing an acceptable casting typically required constructing a mold and attempting to fill it with the molten metal using the casting process of choice. If an excessive amount of gas was entrapped with the solidified metal, then efforts would be needed to reduce this amount by either varying the rates of a molten metal flowing into the mold cavity, or in vacuum casting processes, by varying the vacuum levels applied to the mold. If these variations proved to be unsuccessful, then the more costly alternative of modifying the design of the runners and of the casting shape itself would be necessary.
Therefore, the optimization methods by using computational melt analysis, instead of manufacturing physical mold, have lately attracted considerable attention. A method of computational optimization consists of plural steps, such as assigning mold properties, simulating, filling processes, and detecting the presence or absence of entrapped gas. Mathematical simulations of mold filling processes are usually accomplished via finite difference calculations, in which a mold and a mold cavity are divided by an orthogonal mesh of difference cells (see FIG. 2). A curved boundary 11 is represented by a stair-step grouping of cells 12, and flow calculations are performed only for cells inside the boundary 12. It means that difference cells can exactly represent the mold with straight line boundaries, but they are unsuitable for a mold cavity with curved boundaries. Although the precision of a finite difference method would be increased by using smaller cells, the calculation time will increase much faster than the increase in the number of cells.
Another problem is that, the simulation methods presented previously do not take into consideration the velocity of gas flow through the permeable sand mold to outside but include only a melt flow into a calculation. However, in a case where the melt flow is controlled with the permeability of the mold varied, the speed of a gas flowing out of the mold greatly affects the flow rate of the melt in the mold cavity. Further, in the case of the vacuum casting method, the flow rate of the melt is determined by the rate of a gas flowing out of the mold. That is, the gas flow is considered critical specially in the vacuum casting method using a permeable sand mold.